The disclosure of Japanese Patent Application No. HEI 11-006375 filed on Jan. 13, 1999, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to a reformer for reforming a hydrocarbon raw material such as methane, methanol and the like, to produce hydrogen. More particularly, the present invention relates to an improvement in a reformer that combines a steam reforming method with a partial oxidation reforming method. The present invention also relates to a reforming method and to a fuel cell system.
2. Description of Related Art
In general, a fuel cell is a device that uses oxygen and hydrogen as fuels and converts the chemical energy contained in these fuels directly into electric energy without converting it into thermal energy. The fuel cell has an excellent characteristic in terms of the environment, and is capable of achieving a high energy efficiency. Therefore, the development of a fuel cell as a future energy supplying system has been under way extensively.
When the aforementioned fuel cell is used as a generator, in terms of economy, it would be ideal to use natural gas, naphtha and an alcohol component which is methanol, as fuel gases containing hydrogen. However, at the moment, there is no fuel cell that can efficiently generate electricity by directly supplying these fuel gases to a battery. For this reason, many current fuel batteries are equipped with a reformer for producing a fuel gas from a hydrocarbon raw material. In this reformer, the aforementioned natural gas, methanol and the like are reformed so as to produce a fuel gas that is supplied to an anode.
One of the reforming methods in such a reformer is a steam reforming method. In this steam reforming method, a hydrocarbon raw material is reformed by means of steam so as to produce hydrogen. As one example, a chemical reaction (1) that occurs when performing steam reformation using methanol as a raw material is shown below.
CH3OH+H2Oxe2x86x923H2+CO2xe2x88x9249.5(kJ/mol)xe2x80x83xe2x80x83(1)
As can be seen from the formula shown above, this steam reforming reaction is an endothermic reaction. Thus, when performing steam reformation, a burner, a heater and the like are additionally installed in the reformer so as to supply an amount of heat required for the reforming reaction. However, providing the reformer with a burner and a heater increases the size and complexity of the reforming portion, which is problematic.
Hence, a reformer that can eliminate such a problem has been developed as disclosed in Japanese Patent Application Laid-Open No. HEI 9-315801. This reformer additionally makes use of a partial oxidation reforming reaction, which is a raw material reforming method different from the aforementioned steam reforming method. In a partial oxidation reforming method, a hydrocarbon raw material such as methanol is partially oxidized by means of oxygen so as to produce hydrogen and a great amount of reaction heat, which is discharged at the same time. A reaction (2) wherein methanol is subjected to partial oxidation reformation so as to produce hydrogen is shown below.
CH3OH+xc2xdO2xe2x86x922H2+CO2+189.5(kJ/mol)xe2x80x83xe2x80x83(2)
According to this reformer, the partial oxidation reformation that produces such a great amount of heat is performed in a single reformer, thereby supplying an amount of heat required for steam reformation. In such a system, since there is no need to additionally install a heating device, it is possible to prevent size increase and complexity of the device. Further, since thermal energy is produced within the reformer instead of being produced outside the reformer, the amount of heat absorbed by an outer wall and the like of the reformer can be reduced. Therefore, the energy efficiency is also improved. Furthermore, the partial oxidation reformation not only supplies an amount of heat required for steam reformation but also produces hydrogen at the same time. Thus, the partial oxidation reformation is also advantageous in producing hydrogen.
The aforementioned partial oxidation reforming reaction and steam reforming reaction are equilibrium reactions, which depend on temperature. That is, these reactions proceed in a direction that produces hydrogen if the temperature rises, and cause a reaction that restores a raw material from hydrogen if the temperature falls. Hence, in order to cause these equilibrium reactions to proceed in the direction that produces hydrogen, the temperatures of the respective reactions need to be raised to suitable reaction temperatures.
However, in the case of the previously described reformer that combines steam reformation with partial oxidation reformation, the steam reforming reaction and the partial oxidation reforming reaction are performed randomly in the reformer. In such a case, the heat generated through the partial oxidation reformation is homogenized in the entire reformer and absorbed by the steam reformation that is occurring nearby. For this reason, the previously employed reformer needs to be maintained at a high temperature to enhance the efficiency of both the reactions, for example, at 700xc2x0 C. or 800xc2x0 C.
In addition, in order to maintain the interior of the reformer at a high temperature, an expensive heat-resistant material needs to be used for an outer wall member constituting the reformer, which makes an increase in cost inevitable.
The present invention has been made in light of the aforementioned problems. It is an object of the present invention to enhance an efficiency of reformation in a reformer that combines steam reformation with partial oxidation reformation, while maintaining an outer wall at a relatively low temperature.
In order to achieve the above and/or other objects, according to a first aspect of the present invention, there is provided a reformer that combines a steam reforming reaction, in which a hydrocarbon raw material is reformed into hydrogen using steam, with a partial oxidation reforming reaction, in which a hydrocarbon raw material is reformed into hydrogen using oxygen. In a chamber of the reformer, an oxygen supply amount is reduced from a center of the chamber toward an inner surface of an outer wall of the chamber, and a steam supply amount is reduced from the inner surface of the outer wall toward the center. Thus, a partial oxidation reaction takes place in the vicinity of a central area of the chamber, whereas steam reformation takes place in the vicinity of an outer peripheral area of the chamber, which surrounds the central area.
According to the aforementioned aspect of the invention, the reaction heat generated through the partial oxidation reformation, which is an exothermic reaction performed in the central area, is utilized by the steam reforming reaction in the outer peripheral area. Hence, a temperature distribution pattern is formed in the reformer, and the temperature gradually becomes lower from the central portion of the chamber toward the inner surface of the outer wall of the chamber. Thus, it becomes possible to keep a temperature of the outer wall low. In other words, when the temperature of the outer wall needs to be maintained at the same reaction temperature as in the related art, for example, at 700xc2x0 C., the reformer of the present invention makes it possible to maintain the central portion at a higher temperature. As a result, the efficiency of the reforming reaction is improved. Also, in order to achieve the same reaction efficiency as in the related art, the reforming reaction can be performed with the temperature of the outer wall being kept lower in comparison with the outer wall temperature of the related art.
Further, according to the aforementioned aspect of the invention, an oxygen supply may include a blowout portion for supplying oxygen through blowout in the vicinity of the center of the chamber, and oxygen may be supplied from the vicinity of the center of the chamber toward the inner surface of the outer wall.
According to the aforementioned aspect of the invention, oxygen is blown out from the blowout portion, whereby it becomes possible to securely form a gradient of an oxygen supply amount from the center of the chamber toward the inner surface of the outer wall. Thus, the partial oxidation reformation is efficiently performed in the vicinity of the center of the reformer chamber. On the other hand, it is possible to gradually reduce the ratio of the partial oxidation reformation and increase the ratio of the steam reformation toward the inner surface of the outer wall (i.e., toward the outer periphery of the chamber). Further, since the blowout portion is located at the center, a stream from the center toward the inner surface of the outer wall is formed when supplying oxygen from the blowout portion. This stream shifts the reaction heat toward the outer peripheral area, and this reaction heat is efficiently utilized for the steam reforming reaction that takes place in the outer peripheral area.
Furthermore, according to the aforementioned aspect of the invention, steam of a high partial pressure may be supplied to the vicinity of the inner surface of the outer wall.
In the outer peripheral area where the steam reformation is performed, the more distant an area is located from the central area where the partial oxidation reformation is performed, the more the temperature and the efficiency of the steam reformation in that area tend to fall. However, according to an aspect of the present invention, the partial pressure of the steam in the vicinity of the inner surface of the outer wall is enhanced, whereby the equilibrium reaction of steam reformation proceeds in such a direction as to produce hydrogen. Thus, even if the temperature has fallen, it is possible to achieve a high reformation rate.
Further, according to a reforming method of an aspect of the present invention, in a reformer having an internal chamber formed by an outer wall, a steam reforming reaction in which a hydrocarbon raw material is reformed into hydrogen using steam and a partial oxidation reforming reaction in which a hydrocarbon raw material is reformed into hydrogen using oxygen are performed. According to this method, in the internal chamber of the reformer, the amount of oxygen is reduced from the center of the internal chamber toward the inner surface of the outer wall, whereas the amount of steam is reduced from the inner surface of the outer wall toward the center of the chamber. Thus, the partial oxidation reforming reaction is performed in the vicinity of the central area of the internal chamber, whereas the steam reforming reaction is performed in the vicinity of the outer periphery of the internal chamber.
According to the aforementioned aspect of the invention, the reaction heat generated through the partial oxidation reformation, which is an exothermic reaction performed in the central area of the internal chamber, is utilized for the steam reforming reaction performed in the vicinity of the outer periphery of the internal chamber. Thus, a certain temperature distribution pattern is formed in the reformer. The temperature gradually becomes lower from the central portion of the internal chamber toward the inner surface of the outer wall, whereby it becomes possible to maintain the outer wall at a relatively low temperature.
In the reformer and reforming method of this aspect of the present invention, the partial oxidation reaction is performed in the vicinity of the central area, and the steam reformation is performed in the vicinity of the outer periphery of the chamber, which surrounds the central area. However, some partial oxidation reactions also require steam. Therefore, in addition to the partial oxidation reaction, the steam reforming reaction may partially be performed in the vicinity of the central area of the chamber.
A fuel cell system of an aspect of the present invention includes the reformer described above and a fuel cell that generates electricity by being supplied with reformed gas that is produced through a reforming reaction in the reformer. According to such a fuel cell system, the steam reforming reaction, which is an endothermic reaction, is performed with the aid of the heat generated through the partial oxidation reforming reaction of the hydrocarbon raw material. Therefore, it is possible to reduce an amount of heat that is supplied from outside of the fuel cell system, so as to supply an amount of heat required for the reforming reaction. Thus, there is no need to provide a source of heat for supplying an amount of heat required for the reforming reaction. In addition, in the reformer used in the fuel cell system, the partial oxidation reaction, which is an exothermic reaction, is performed in the central area of the chamber, and the steam reforming reaction, which is an endothermic reaction, is performed in the outer periphery of the chamber. Thereby, the reaction heat generated in the central area is absorbed as an amount of heat required for the steam reforming reaction when diffusing toward the inner surface of the outer chamber wall. Hence, it becomes possible to maintain the outer periphery at a relatively low temperature. Accordingly, without using an expensive heat-resistant material for the outer wall, a certain temperature distribution pattern is formed in the reformer, so that the efficiency of the reforming reaction can be improved. Thus, the fuel cell system of the present invention makes it possible to prevent complexity of the entire system and to enhance the energy efficiency.